In particular, in patients with hip dysplasia, the existing deformities of the pelvic bone make ideal positioning of a hip joint endoprosthesis difficult. Since it is not possible to ream the existing pathological acetabulum as cup bearing, an approximately anatomical cup bearing must be reconstructed. This may, however, deviate significantly from the existing pathological situation. Although an optimal position can be planned preoperatively, the surgeon has no possibility intraoperatively of transferring the visual plan in a defined manner. Here he has to rely essentially on his experience.